A video signal can be transmitted in data packets over a network from a transmitting device to a receiving device. A video signal may comprise a sequence of video frames which are to be played out in order to thereby output the video signal. Each frame may comprise a plurality of blocks of pixels (or “macroblocks”) which are coded separately. A block may be intra coded meaning that the block is coded without reference to data in other frames of the video signal. Alternatively a block may be inter coded meaning that the block is coded with reference to data in other frames (“reference frames”) of the video signal. An inter coded block may be associated with a motion vector to describe motion in the image between the block and a corresponding position in the reference frame.
Errors may occur during the transmission of the video signal, for example a data packet may be lost or corrupted during transmission. When blocks are inter coded, errors may propagate through a sequence of frames of the video signal. Errors may be caused by conditions on the network on which the video signal is transmitted. As an example, network conditions may vary for receiving devices of individual users on a network (e.g. the internet) such that the errors experienced by each individual user may be different to other users. It can be useful to monitor quality of the video signal received at each of the receiving devices. Three methods of assessing video quality are:                1. Full Reference (FR) methods which evaluate video quality in a processed video signal (e.g. a received video signal) by comparing an original video signal (e.g. a transmitted video signal) and the processed video signal;        2. Reduced Reference (RR) methods which evaluate video quality in a processed video signal (e.g. a received video signal) using both the processed video signal and a small amount of information extracted from the original video signal (e.g. the transmitted signal); and        3. No Reference (NR) methods which evaluate video quality in a processed video signal (e.g. a received video signal) on the basis of the processed video signal alone (i.e. without reference to the original video signal).        
The No Reference methods are particularly useful in situations where a video signal is being transmitted from a transmitter over a network to a receiver because these methods allow the receiver to estimate the quality of the video signal without needing knowledge of the original (i.e. transmitted) video signal. Therefore it is not necessary to transmit any extra information to the receiver regarding the original video signal for specific use by the No Reference method. This is in contrast to the Full Reference and Reduced Reference methods for which the receiver would be required to have extra information regarding the original video signal in order to estimate the quality of the received video signal. Therefore, desirably the No Reference methods require less information to be transmitted from the transmitter to the receiver in order for the receiver to be able to estimate the quality of the received video signal, and therefore require less bandwidth on the network.
A No Reference method for estimating video quality may do so on the basis of the number of macroblocks containing errors which it has not been possible to conceal. Error concealment effectiveness is evaluated using motion-level information and luminance discontinuity at the boundaries of error regions. Motion information can be obtained from motion vectors in an input bitstream. If the motion vectors indicate a motion that is greater than a pre-determined threshold then error concealment is considered to be ineffective. Furthermore, the luminance discontinuity is calculated as an average of the absolute difference of the luminance values at the boundary between correctly decoded regions and regions to which error-concealment has been applied. If the luminance discontinuity is larger than a pre-determined threshold then error concealment is considered to be ineffective. The No Reference method of the prior art described above provides one way of estimating the quality of a received video signal at a receiving device. However, the method is not perfect because the number of macroblocks containing errors which it has not been possible to conceal may not always provide an accurate measure of the quality of the video signal and the method increases the decoding time required to decode a received video signal.